1. Technical Field
The present invention relates to CMOS imagers and more particularly to a structure of microlens array of CMOS imager and to a method for manufacturing this microlens array.
2. Description of the Related Art
Imagers manufactured according to the CMOS technology (“Complementary Metal Oxide Semiconductor”) are currently used in an increasing number of applications due to their low cost price compared to CDD imagers (Charge Coupled Device). These CMOS imagers were initially used to manufacture low-resolution image sensors of poor quality (such as webcams). Today, after a significant investment in Research and Development, CMOS imagers can rival with CDD imagers. The present invention is part of the effort of improving this imager technology.
FIG. 1 shows an example module of picture and/or video frame grabbing using a CMOS imager, intended for example to be built into a portable device such as a mobile phone, a camera or a video camera. The module 1 comprises a frame 2, a lens holder block 3, lenses 4 attached to the block 3, an infrared filter 5 and a support 6. A CMOS imager 10 is arranged on the support 6 and receives the light passing through the microlenses and the infrared filter.
The CMOS imager 10 comes under the form of a semiconductor chip and comprises several photosites, each forming a pixel (not visible on FIG. 1). Each pixel comprises a photodiode and a circuit for controlling and interconnecting the photodiode. The pixels are arranged in a matrix way and a mosaic of red, green, blue filters spreads above the matrix of pixels, generally according to the Bayer architecture (the cells of a row being alternately red and green or alternately green and blue). Each pixel is thus covered by a filter of determined primary color—red, green or blue—and provides luminance information relating to the primary color allotted thereto, forming pixel information.
FIG. 2 is a schematic cross-sectional view of the CMOS imager 10 in a region corresponding to three pixels PIX1, PIX2, PIX3. From the bottom to the top, layers 11, 12, 13, 14, 15 and microlenses L0 (L0-1, L0-2, L0-3) can be seen. The layer 11 is the semiconductor substrate on which the imager is implanted. This layer 11 thus represents the active part of the imager and comprises photodiodes and their associated control and interconnection circuits (not detailed). The layer 12 is formed by a dielectric material which totally covers the substrate 11. The layer 13 is a passivation layer deposited on the imager at the end of the CMOS manufacturing process. The layer 14 is formed by colored resins and comprises red, green, or blue sectors 14-1, 14-2, 14-3 forming the aforementioned filters of primary color, on the basis of one colored filter by pixel. The layer 15 is an intermediary layer of resin forming a support for the microlenses L0 and offering a good flatness. The microlenses L0 are arranged in a Microlens Array called “MLA” on the basis of one microlens by pixel.
FIG. 3 is an exploded cross-sectional view of the imager 10 showing the structure of a pixel PIXi. The colored filter 14 and the microlens L0-i of the pixel are represented far from an active part 16 which is represented without its dielectric material so as to show components comprised therein. Thus, a photodiode 121 doped n+ formed above a well 11′ doped d implanted in the substrate 11 is shown, together with elements forming the circuit for controlling and interconnecting the photodiode. These elements for example comprise an amplifier transistor 122, a column selection bus transistor 123, a reset transistor 124, and a row selection bus 125.
A distinctive feature of CMOS imagers, which is shown in FIG. 3, is that the photodiode occupies only a part of the total surface of the pixel, the remainder being occupied by the circuit for controlling and interconnecting the photodiode. For that reason, a CMOS pixel is generally called “active pixel”, conversely to a pixel of CCD imager where the photodiode substantially occupies all the surface of a pixel. In practice, the photodiode generally occupies 50% of the surface of the pixel only.
The microlens L0 is used to collect and to focus on the photodiode 121 the photons received by the pixel. Without microlens, the yield of the imager (ratio between the received light energy and the light energy collected and transformed into electric voltage) would be poor and the supplied images would have a low brightness and a low contrast. Thus, the “fill factor” refers to the percentage between the effective surface of the pixel (surface of the photodiode) and the total surface of the pixel. The provision of a microlens array allows a higher fill factor to be obtained. The fill factor then corresponds to the ratio between the surface occupied by the microlenses and total surface of the active part of the imager, because all the light collected by the microlenses is assumed to be sent on the photodiodes.
A conventional structure of microlens array L0 is shown in top view in FIG. 4. The microlenses have a circular base and a constant diameter, and are spaced out between them by a centre-to-centre distance Pch called “pitch”, corresponding to the pitch of the photodiode array. The nearest edges of the microlenses do not touch and are at a distance ∈. This distance is generally reduced to a minimum ∈min offered by the manufacturing method. By way of example, with the current photolithography methods, the minimum separating distance ∈min that must be respected is 0.4 μm. Thus, for a pitch of 4 mm, the maximum diameter of a microlens that can be chosen is 3.6 μm. Mathematically, taking into account the circular shape of the base of the microlenses, the fill factor obtained is therefore about 64%.
Thus, despite the provision of the microlens array, 36% of the surface of each pixel is lost, i.e. 36% of the total surface of the imager. This drawback is due to the circular shape of the base of the microlenses but also to the distance ∈min between the edges of the adjacent microlenses.
The conventional manufacturing method of this microlens structure is shown in FIGS. 5A to 5E and comprises the following steps:                depositing a layer of photosensitive polymer resin 21 (photoresin) on an imager wafer 20 and soft bake of the layer of resin (FIG. 5A),        exposing the layer of resin 21 to an ultraviolet light through an insulation mask M0 (FIG. 5B),        removing insulated parts from the layer of resin 21 with an organic solvent (FIG. 5C), to obtain an array of flat pellets P0,        thermal creep of the pellets P0 to obtain microlenses L0 with a convex upper face (FIG. 5D),        afterbake of the microlenses L0 to ensure their hardening (FIG. 5E).        
The resin used is a positive resin, i.e. having a high solubility in presence of an adapted solvent (etching agent) after UV exposure. The insulation mask M0 thus has dark areas which shape is identical to the shape of the microlenses to be made, and transparent areas spreading between the dark areas, corresponding to the areas of resin to be removed. The minimum separating distance ∈min between the edges of the microlenses L0 corresponds to the minimum distance between the dark areas of the mask M0. Below this minimum distance, flashes appear on the edges of the pellets P0 which do not separate properly, causing distortions of microlenses shapes after the creeping step.
In conclusion, the drawback of the above described conventional structure of microlenses is to have a fill factor far from the 100% ideal value, on the one hand because of the circular shape of the microlenses that limit filling in, and on the other hand because of the minimum separating distance ∈min between the edges of microlenses.